add "ext-info-in-auth@openssh.com" extension

add "ext-info-in-auth@openssh.com" extension

This adds another transport protocol extension to allow a sshd to send
SSH2_MSG_EXT_INFO during user authentication, after the server has
learned the username that is being logged in to.

This lets sshd to update the acceptable signature algoritms for public
key authentication, and allows these to be varied via sshd_config(5)
"Match" directives, which are evaluated after the server learns the
username being authenticated.

Full details in the PROTOCOL file